Method for electrostatically coating the surfaces of articles with pulverulent materials

ABSTRACT

A METHOD FOR ELECTROSTATICALLY COATING THE SURFACES OF ARTICLES WITH PULVERULENT MATERIALS, ESPECIALLY COLORING SUBSTANCES, BY SPRAYING THE MATERIAL DISPERSED IN AN AIR CURRENT UPON THE ARTICLE AND PERMITTING THE RECOVERY OF THE EXCESSIVE MATERIAL WHICH IS DRAWN OFF BY SUCTION FROM THE SPRAY CHAMBER, TREATED, AND THEN RETURNED TO THE SUPPLY FOR IMMEDIATE REUSE IN THE SPRAYING OPERATION.

Sept. 26, 1972 M. OFNER 3,694,242

METHOD FOR ELECTROSTATICALLY COATING THE SURFACES 0F ARTICLES WITHPULVERULENT MATERIALS Original Filed July 26, 1967 4 Sheets-Sheet 1 26 mII: III 2 I l ETE /n ven tor:

Max OFNER his AHorney Sept. 26, 1972 M. OFNER 3,694,242

METHOD FOR ELECTHOSTATICALLY COATING THE SURFACES 0F ARTICLES WITHPULVERULENT MATERIALS Original Filed July 26, 1967 4 Sheets-Sheet 2Inventor: Max OFNER his Aflorney Sept. 26, OFNER METHOD FORELECTROSTATICALLY COATING THE SURFACES OF ARTICLES WITH PULVERULENTMATERIALS Original Filed July 26, 1967 4 Sheets-Sheet 5 /0/ z /E0 1 105i W lnventor:

Max DFNER 1, mm 0% his Afiorney P 1972 M. OFNER 3,694,242

METHOD FOR ELECTROSTATICALLY COATING THE SURFACES OF ARTICLES WITHPULVERULENT MATERIALS Original Filed July 26, 1.96"

4 Sheets-Sheet 4 r R w w w nNn m m b m m M m M m a United States Patent3,694,242 METHOD FOR ELECTROSTATICALLY COATING THE SURFACES 0F ARTICLESWITH PULVERU- LENT MATERIALS Max Ofner, Leonberg, near Stuttgart,Germany, assignor to SP3 Spezial-Filterbau M. Ofner K.G., Friolzheim,Wurttemberg, Germany Continuation of abandoned application Ser. No.656,081, July 26, 1967. This application Jan. 5, 1971, Ser. No. 104,138Claims priority, application Germany, July 29, 1966, P 16 96 368.1; May5, 1967, P 16 46 104.4 Int. Cl. B05b 5/02; B44d 1/094 U.S. Cl. 117-17 12Claims ABSTRACT OF THE DISCLOSURE A method for electrostatically coatingthe surfaces of articles with pulverulent materials, especially coloringsubstances, by spraying the material dispersed in an air current uponthe article and permitting the recovery of the excessive material whichis drawn off by suction from the spray chamber, treated, and thenreturned to the supply for immediate reuse in the spraying operation.

This application forms a continuation application of application Ser.No. 656,081, filed July 26, 1967, and now abandoned.

The present invention relates to a method for electrostatically coatingthe surfaces of articles with pulverulent materials, especially coloringsubstances, wherein the material which is supplied from a container anddispersed in an air current is electrically charged and sprayed upon thearticle to be coated and the excessive material is removed by suction.The invention further relates to an apparatus for carrying out thismethod.

The known methods of the above-mentioned type have two seriousdisadvantages, namely, that they do not permit the application ofmaterials the particles of which have the tendency to stick to eachother and to form lumps which will prevent the material from beingproperly sprayed and will clog the spraying device, and that the amountof material which during the spraying does not reach the article to becoated and will be sucked otf is relatively large as compared with thetotal amount of the sprayed material. Since the material which isremoved by suction can no longer be used because of the practicallyunavoidable formation of lumps, this method is very uneconomical,especially when the materials used, for example, epoxide resins, arerelatively expensive.

It is an object of the present invention to provide a method of the typeas first described in which the above mentioned disadvantages will notoccur. This is attained according to the invention by permitting thelarger as well as smaller particles of the excess material which aredrawn otf by suction to be recovered for further use by being separatedfrom the air current coming from the suction line and by eliminating theelectric charge of the entire recovered material and screening thelatter before it is sprayed again.

Since the present invention permits the recovery of all the particles ofthe material which during the spraying operation do not reach thearticle to be coated, there will be practically no loss of material. Therecovery of this material has the further advantage that the coatingoperation may be carried out at a greater speed since the usual spraygun which is employed for the spraying operation no longer has to bepointed at all times very exactly upon the article to be coated as wasnecessary in the known methods which did not provide for any recovery ofthe Patented Sept. 26, 1972 'ice excess material in order to reduce theloss of material as much as possible. Since the electrostatic charge ofthe material which is drawn off by suction is removed and this materialis screened and if necessary disintegrated, it will be treated so as topermit its reuse without danger that any clogging or other breakdown ofthe spraying apparatus may occur.

In order to prevent any interference in the proper operation of thespraying apparatus it is advisable to keep the larger and smallerparticles of the recovered material which has been separated from thesuction current in a mixed condition. This has also the advantage thatno rough spots will be formed on the surface of the coated article. Thisresult which by the known methods could not be attained is surprisinginsofar as it was previously the prevailing opinion that if the excessmaterial was recovered it could be used again only if the fine particleswere removed because of the dust which might be intermixed with them.

The larger and smaller particles of the material may be kept in a mixedcondition by holding them in suspension by means of air. According to avery advantageous embodiment of the inventive method, the air which isrequired for this purpose is conducted into the supply container of thematerial by passing it through a porous layer on the bottom of thiscontainer.

When employing materials which have the tendency to form lumps, it isadvisable also to subject the new material to a disintegrating andscreening process before it is sprayed. Lumps or larger particles whichmight have been formed during storage of the material will thus beeliminated so that all possible causes of interferences in the sprayingapparatus will be avoided.

According to a preferred embodiment of the invention, the screenedmaterial is conducted back into the supply container from which thematerial is taken for the spraying operation. Thus, a closed circuit isformed which insures a very economic operation since it is then onlynecessary to pass approximately the same amount of material into thecirculation which is required for the actual coating process. A changefrom one material to another may then also be carried out in a verysimple manner and will not lead to any noticeable loss of material.

For attaining the further object of the invention to provide anapparatus for carrying out the new method, this apparatus comprises aspraying device for ejecting the electrically charged particles of thematerial within an air current, a suction device for drawing off theexcessive material, and a separating device following the suction devicefor segregating the excessive particles and collecting them in acontainer, wherein this separating device is provided with a filter forseparating and recovering the fine particles which might still becontained in the air which is discharged from the separating device. Theobject of the invention is further attained by designing the containerin which the separated particles including those which are separated bythe filter are collected so that the surfaces of this container withwhich the particles come in contact form a discharge electrode forremoving the electric charge from these particles, and by providingsuitable means for disintegrating and screening the material which iscollected in this container.

The separating device of the new apparatus is preferably provided in theform of a cyclone separator and the filter is preferably mounted in sucha manner within the path of the air which is discharged from thisseparator that the particles which are separated from this air by thefilter will pass into the collecting container which re ceives theparticles which are separated by the cyclone separator. This arrangementinsures that not only the larger particles which are separated from theair by the cyclone separator but also the smaller particles which aresubsequently separated from the exhaust air by the filter will pass intothe same container and will therefore remain as an intimate mixture witheach other.

In an apparatus which forms a very advantageous embodiment of theinvention, the filter is built into the mentioned collecting containeror into the cyclone separator which is located above this container. Theparticles which are separated by the filter then do not need any meansfor conveying them into the collecting container since they will fallinto the latter due to their own gravity. This construction of theapparatus has the further advantage that it has a relatively low height.If desired, it is, however, also possible to mount the filter on top ofthe separating device. If this apparatus should have a lower height, itis possible to provide it with two lower filters which are connected inparallel.

The disintegrating and screening device is preferably provided in theform of a sifter of a conventional construction in which the material isconveyed by a rotating worm which throws it at the same time against asurrounding screen of a cylindrical shape.

The collecting container is preferably mounted directly above the inletopening of the disintegrating and screening device so that no additionalconveying means will be required to pass the material from one of theseunits to the other. In addition it is advisable to provide the apparatusbetween the collecting container and the disintegrating and screeningdevice with a dosing device, for example, a rotatable gate wheel with aplurality of compartments, the operation of which is adapted to becontrolled so as to permit the amount of material to be varied which ispassed to the disintegrating and screening device.

Another advantageous feature of the invention consists in providing theoutlet channel of the collecting container to the gate wheel with aplurality of peripherally spaced nozzles which are adapted to blowblasts of compressed air at predetermined intervals into this channel soas to prevent the material from becoming condensed within the outlet ofthe collecting container and from clogging this outlet. These nozzlesmay be arranged so as to blow the air upwardly and at an oblique angleinwardly into the cutlet channel of the collecting container.

In place of such nozzles, it is another feature of the invention toprovide the collecting container with a porous layer at least on theinner side of the bottom of this container adjacent to its outlet to thegate wheel, and with a line for passing compressed air through thislayer into the collecting container so as to produce therein asuspension of air and the particles of the material which insures thatall particles will be uniformly mixed with each other and also that aformation of lumps or a clogging of the outlet of the collectingcontainer will be prevented. The apparatus is further preferablydesigned so that not only the recovered material is passed into thiscollecting container so as to insure that an intimate mixture betweenthe recovered and the new material will be formed.

In connection with such a porous layer it is another fea-v ture of theinvention to cover at least the inner side of this layer with anelectrically conductive coating. This has the advantage that also thepart of the collecting or supply container which is covered by theporous layer will serve as a discharge electrode for removing theelectric charge from the particles of the material.

The spray chamber which is preferably provided with a plurality ofsuction inlet openings of the suction device may either be mounted at aplace separate from the apparatus itself or it may be combined therewithby being attached to one side thereof.

The various features and advantages of the present invention will becomemore clearly apparent from the detailed description thereof which is tobe read with reference to the accompanying drawings, in which- FIG. 1shows a front view of an apparatus according to a first embodiment ofthe invention;

FIG. 2 shows a side view of the same apparatus;

FIG. 3 shows a top view of the apparatus according to FIGS. 1 and 2;

FIG. 4 shows a front view of an apparatus according to a secondembodiment of the invention;

FIG. 5 shows a cross section which is taken along the line V-V of FIG.4;

FIG. 6 shows a cross section of the bottom of the collecting containerof the apparatus according to FIGS. 4 and 5; while FIG. 7 shows a frontview of an apparatus according to a third embodiment of the invention.

Referring first to FIGS. 1 to 3 of the drawings, the apparatus accordingto the invention comprises a frame consisting of a supporting plate 1and feet 2 on which a collecting container 3 is mounted which iscomposed of an upper cylindrical vessel 4 and a lower funnellike supplycontainer 5 which is directly connected to this vessel 4. The collectingcontainer 3 is symmetrical about its vertical axis and mounted on thesupporting plate 1 in such a manner that the vessel 4 is located abovethis plate 1 and the lower end of the funnellike supply container 5which is provided with an outlet channel 6 is located between plate 1and the floor on which the feet 2 are resting. The conical lowercontainer part 5 has an angle of approximately Near its point ofconnection to the outlet channel 6, the lower container part is providedwith a plurality of peripherally spaced inclined nozzles 7 which areconnected to each other by an annular line and are adapted to passblasts of compressed air at predetermined intervals into the outletchannel 6.

As indicated in FIG. 2 an outlet tube 11 of a blower 12 which is mountedon the supporting plate 1 extends tangentially and at a slight downwardinclination into the vessel 4. This blower 12 which is driven by anelectric motor 13 forms together with the vessel 4 a cyclone separator.A suction line 14 leads from the blower 12 to a suction channel 15 whichis provided with openings 15 and is located adjacent to the frame 1, 2on the bottom of a spray chamber 17 and extends across the entire widthof the frame. As shown in FIG. 1, a second suction line 18 which has aconsiderably smaller diameter than the suction line 14 extends to thearea underneath the supporting plate 1 and its end may be inserted intoor connected to a container in which the new material to be sprayed isheld for conveying this material into vessel 4.

The vessel 4 carries on its upper side two filters 8 and 9 which areconnected in parallel and through which the exhaust air of the cycloneseparator is conducted. These filters 8 and 9 are designed so that theparticles of the material which are separated by them from the exhaustair of the cyclone separator will fall into the supply container 5. Inorder to permit the inside of the container 3 to be cleaned whennecessary, the upper wall of vessel 4 1s provided with a manhole 10which may be closed by a cover.

The outlet channel 6 is connected by a gate wheel 19 with a plurality ofseparate compartments to the inlet of a conventional sifter 21. Gatewheel 19 which is driven by an electric motor 20 is adapted at everyrevolution to pass a certain quantity of the material from thecollecting container 5 into the sifter 21. This sifter is provided witha rotating feed worm for conveying the material and for throwing itagainst a resilient screen surrounding it. As illustrated in FIG. 2, thesifter 21 is provided with two separate outlets 22 and 23 for thetreated material and for any waste which has been screened out and is tobe discarded.

The outlet 22 for the treated material terminates into a supplycontainer 24 of a pneumatic conveying device which conducts the materialwhich is dispersed within an air current through a connecting line 25 toa spray gun 26 of a conventional construction which is provided with anatomizer nozzle and an electrode for electrically charging the particlesof the material which emerge from this nozzle.

The mode of operation of the method and apparatus according to theinvention which may be employed, for example, for treating and sprayingepoxide resins is as follows:

The amount of material which is required for the particular coatingoperation is conveyed through the second suction line 18 into thecontainer 3. Thereupon the sifter 21 and thereafter the gate wheel 19are started. Gate wheel 19 conveys the material in measured quantitiesinto the sifter 21. The material is supplied from the container to thegate wheel 19 automatically by its own gravity, while the nozzle 7prevents the material from being packed in the outlet channel and fromclogging the same.

After entering the sifter 21, the material is disintegrated thereininsofar as it may be necessary and screened. After this treatment, thematerial emerges from the outlet 22 and passes into the storagecontainer 24 of the pneumatic conveying device. The operation of thegate wheel 19 may be controlled so that the quantity of material whichis contained in this storage container will always remain between acertain maximum and minimum amount.

After entering the container 24 of the pneumatic conveying device, thematerial to be sprayed is dispersed therein within an air current andconducted through the connecting line 25 to the spray gun 26 in whichthe particles are electrically charged before being ejected. During thespraying operation, the spray gun 26 is directed upon the article to becoated and held at a suitable distance therefrom.

Those parts of the sprayed material which did not settle on and adhereto the article to be coated are drawn off by the suction of the blower12 through the openings 15 of the suction channel 16 and are blown intovessel 4. Due to the downward inclination of the outlet tube 11, theparticles which are blown from this tube receive a motion component inthe downward direction which facilitates their precipitation from theair current. Those parts of the drawn-off material which are not as yetsep arated from the air current are then separated in the filters 8 and9 and fall into the connecting container 3.

The particles of the material which are separated from the exhaust aircome in contact with the wall of the vessel 4 and partly also with thewall of the supply container 5 and thereby lose their electric charge.The large surfaces of the container 3 facilitate the release of theelectric charge of the particles which is then conducted to the ground.If any charge remains in the particles, it will be passed off within thegate wheel 19 and in the sifter 21. Thereafter, the particles of thematerial may again pass through the circuit as above described.

Of course, the outlet 22 of the sifter 21 does not need to be connecteddirectly to the supply container 24 of the pneumatic conveying device,but the treated material coming out of the sifter 21 may be conveyed tothe pneumatic conveying device in any other suitable manner.

While it is advisable, as illustrated in FIGS. 1 and 3, to feed the newmaterial through the line 18 to the container 3 so as to mix it thereinthoroughly with the recovered material coming from the spray chamber 17and also to treat it together with the recovered material, it is alsopossible to omit the treatment of the new material by feeding it to apoint between the container 5 and the gate wheel 19 or to the sifter 21or directly to the container 24 of the pneumatic conveying device inwhich the new material is then thoroughly mixed with the recoveredmaterial before the mixture is electrically charged and sprayed by thespray gun 26.

FIGS. 4 to 6 illustrate another embodiment of the apparatus according tothe invention in which a cyclone separator consisting of a verticalcylindrical vessel 104 and a blower 112 is mounted in the same manner aspreviously described on a frame 101 to which a spray chamber 117 isattached. Of course, this spray chamber 117 may also be mountedseparately from the frame 101. The pressure outlet 111 of the blower 112terminates tangentially into the vessel 104 and the suction inlet ofblower 112 is connected by a connecting line 114 to the inside of thespray chamber 117.

Vessel 104 is directly connected underneath to a supply container whichtogether with the vessel 104 forms the main collecting container 103.The supply container 105 consists of two coaxial cylindrical sections105a and 10Sb and an intermediate conical section 1050. Sections 105awhich is directly connected to the vessel 104 has a diameterconsiderably larger than that of the section 1051) but equal to thediameter of the vessel 104.

The supply container 105 is closed at its lower side by a bottom whichhas a central outlet channel 106. As shown in FIG. 6, the containerbottom 130 is provided on the inside with a porous layer 132 which isspaced from the outer wall 131 and secured thereto by spacing elements133. The space between the porous layer 132 and the outer wall 131 formsa chamber which communicates with a compressed-air supply line 134 whichterminates into this chamber within the level of the outlet channel 106.

On its inner side, the porous layer 132 which consists of a syntheticmaterial is provided with an elec trically conductive coating 135 whichdoes not affect the porosity of the layer 132 and participates incompletely removing the electric charge from the recovered particles.

The upper section 105a of the supply container 105 contains an airfilter 108 from which the discharged air emerges from the container 105to the outside through an outlet 136.

The powder to be treated may be passed into the supply container 105through an opening (not shown) in the cover 136' of the vessel 104 or bymeans of a pneumatic conveying device similar to the conveying device 18which is connected in parallel to the suction line 114.

In all other respects, the apparatus according to FIGS. 4 to 6 isdesigned substantially in the same manner as the apparatus according toFIGS. 1 to 3. The coinciding features therefore do not need to bedescribed again.

Since the compressed air which is supplied through the supply line 134is forced through the porous layer 132 into the supply container 105,the powder which is located in this container forms a kind of asuspension in this air which results in an excellent mixture of theparticles of the material and also prevents the powder from forminglumps which otherwise it has a tendency to do and which might result inthe danger that the outlet channel 106 might become clogged.

The powder is removed from the supply container 105 in the same manneras described with reference to the apparatus according to FIGS. 1 to 3by means of a gate wheel 120 and passed through a sifter 121 to thespraying device. The particles of the material which are drawn by theblower 112 out of the spray chamber 117 are blown tangentially into thevessel 104 of the cyclone separator in which the larger particles areseparated from the air current in a conventional manner. Due theirgravity, these larger particles then drop into the supply container 105,while the fine particles are separated from the air current in aconventional manner. Due to their gravity downwardly into the supplycontainer 105. Since a suspensionlike mixture of air and material isthus formed, the recovered particles will then remain in the supplycontainer in an intimate mixture with the other particles.

FIG. 7 illustrates an apparatus according to a further embodiment of theinvention which differs from the apparatus according to FIGS. 4 to 6only by having a larger frame 201 and by being provided with two equalseparating units 2.03 and 203' which are supported by this frame. Thesetwo separating .units are connected by separate connecting lines 214 and214' to the inside of the spray chamber, and each of these units isfollowed by a sifter 221 and 221' respectively. The mode of operation ofthese two recovery units is the same as described with reference toFIGS. 4 to 6.

The twin apparatus according to FIG. 7 has the advantage that the changefrom the treatment of the material contained in one supply container tothe treatment of the material in the other supply container can becarried out very quickly since it is only necessary to exchange thespray gun and to switch off the blower of one apparatus and to switch onthe blower of the other apparatus.

Although my invention has been illustrated and described with referenceto the preferred embodiments thereof, I wish to have it understood thatit is in no way limited to the details of such embodiments but iscapable of numerous modifications within the scope of the appendedclaims.

Having thus fully disclosed my invention, what I claim is:

1. A method of electrostatically coating the surfaces of a articles withpulverulent materials for producing a uniformly smooth surface thereon,said pulverulent materials having particles of different sizes,comprising the steps of dispersing said material in a compressed-aircurrent and conveying it by said air current to a spraying device,electrically charging said material and spraying said material from saidspraying device upon an article to be coated, drawing off by suction ata suction station substantially all of the excess spraying material notadhering to said article, recovering said excess sprayed material byseparating the larger and smaller particles of the recovered materialfrom the air current coming from said suction station, thoroughly mixingall of the larger and smaller particles of the recovered excess materialafter they have been separated from the air current coming from saidsuction station, substantially eliminating the electric charge from saidrecovered particles, dispersing said thus-treated recovered particles ina compressed-air current and conveying them by said air current to saidspraying device, and then electrically charging said recovered particlesin said spraying device and spraying the same therefrom upon an articleto be coated.

2. A method as defined in claim 1, wherein said larger and smallerparticles after being separated from the air current coming from saidsuction station are thoroughly mixed with each other by maintaining themin a fluidized state by a second current of compressed air.

3. A method as defined in claim 2, wherein said second current ofcompressed air is passed into and mixed with said recovered particles bybeing conducted from the outside through a porous layer covering atleast a part of the inner wall of a container in which said particlesare collected.

4. A method as defined in claim 1, comprising removing in separateoperations the larger and smaller particles of the recovered excesssprayed material from the air current coming from said suction station.

5. A method as defined in claim 4, wherein the separate operations ofremoving the larger and smaller particles from the air current arecarried out at zones in generally vertical alignment, whereby theparticles removed at one such zone fall into at least a portion of 8. Amethod as defined in claim 4 wherein the larger particles are removed ina cyclone separator, and the smaller particles are removed by a porousfilter.

9. A method as defined in claim 8, wherein the porous filter is disposedin alignment with and above at least a portion of the cyclone separatorwhereby the removed smaller particles fall into the lower portion of thecyclone separator.

10. A method as defined in claim 1, comprising supplying new material tothe recovered material, and said recovered material before beingelectrically charged and sprayed.

11. A method as defined in claim 10, wherein the suction for drawing offthe excess material in said section station is also employed forsupplying the new material and for passing said recovered excess and newmaterial together into a common cyclone separator for separating thelarger particles of the mixture, While the remainder is passed throu-gha filter disposed above at least a part of the cyclone separator foralso separating the smaller particles which then fall into the lowerportion of the cyclone separator, then collecting the larger and smallerparticles from the bottom of the cyclone separator by gravity anddispersing them in and mixing them by a second compressed air currentand removing from the mixture the electric charge from the precedingspraying operation, and then screening the mixture and conveyingpredetermined amounts thereof to said spraying device from which themixture after being newly-charged is sprayed.

12. A method of electrostatically coating the surfaces of articles withpulverulent materials, especially coloring substances, comprising thesteps of dispersing said material in a compressed-air current andconveying it by said air current to a spraying device, electricallycharging said material and spraying said material from said sprayingdevice upon an article to be coated, drawing off by suction theexcessive material not adhering to said article in a suction station,and recovering said excessive material by separating the larger andsmaller particles of the recovered material from the air current comingfrom said suction station, substantially eliminating the electric chargefrom said recovering particles and screening the same, dispersing saidrecovered particles in a compressedair current and conveying them bysaid air current to said spraying device and then electrically chargingsaid recovered particles in said spraying device and spraying the sametherefrom upon an article to be coated.

References Cited UNITED STATES PATENTS- 3,377,183 4/1968 Hurt et al.117-19 2,173,032 9/1959 Wintermute 118-312 3,236,565 2/1966 Kester et a1302-17 727,030 5/1903 Tilghman 51-12 2,770,924 11/1956 Mead et a1. 51-83,299,853 1/1967 Whitacre 118-312 2,757,635 8/1956 Lipsius 118-3122,992,126 7/1961 Roberts et a1. 117-17 3,073,070 1/1963 Mead 51-123,167,442 1/1965 Brooks 117-19 3,291,630 12/1966 Deyle et al 117-193,598,626 8/1971 Probst et al. 117-17 3,307,296 3 1967 Ashworth 51-8WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant ExaminerUS. Cl. X.R.

